In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Heat treatment processes require a much more uniform power supply in order to be made cost effective/optimised. The ability to heat-treat goods without subsequent truing/correction of the goods is a must.
The possibility of being able to adjust the properties of materials structurally by heat treatment increases the requirement for uniform power supply.
One example in this regard is found in the production of metal sheeting. Metal sheeting is produced in different thicknesses, widths, lengths and mechanical strength classes, all in accordance with customer's requirements. These product variations result in the inducement of different states of strain by the energy applied.
It can be said in general that an increase in the width of the sheeting lowers production costs and that custom-produced sheet widths increase material costs, although this latter can be compensated for by reducing scrap in the final production.
In order to be able to produce all variants, it is elected to construct the heat treatment equipment for a greatest width.
It is therefore desirable to enable the heat treatment equipment to be provided with a heating system with which the power can be varied transversely of the oven or furnace.
However, a problem arises when it has been elected to provide the heat treatment equipment with radiant tubes in order to maintain a controlled atmosphere.
This problem is normally solved, by installing elements that cover different widths of the furnace space, so that the power developed can be varied in keeping with the product under treatment. This means, however, that it is not possible to install maximum power density in relation to the furnace space delimiting surfaces, measured in kW/m2.
Another way is to use spiral elements that are disposed in different zones which are provided with power outlets that extend coaxially with the axis of the spiral. This element system cannot, however, be dimensioned for a high power output for each radiant tube.
With regard to reliability combined with the possibility of obtaining a high power output with each radiant tube, the type of element that is most attractive is the type normally referred to as a bird cage element, a so called Käfigelement, or bundle rod element sold by Kanthal AB, Sweden, under the name Tubothal.
Traditionally, this type of heating element has, however, been designed as a series-connected element in which the heating filaments, or threads, describe a single loop between two power outlets. Star-connected or delta-connected loops are also used in the case of larger heating elements, wherewith the elements are provided alternatively with three or with four outlets. It is also known to connect the element loop in parallel between two power outlets in the case of applications where a low supply voltage is desired. Cases are also known in which four outlets are used, wherewith it is possible conceivably to connect up a loop that is operative both when desiring a sub-power and a full power.
A common failure with these traditional element constructions, however, is that power cannot be distributed differently along the long axis of the radiant tube. The reason for refraining from providing the heating element with two or more zones along its axis is because the inclusion of additional outlets and connections in the element have a far too great influence on the space available for the heat generating filaments of the element, since these additional outlets must run within the radiant tube, therewith excluding the high power advantage. There is also an increased risk of an electric spark-over, especially when NiCr-type filaments are used.